The present invention is directed to a method and apparatus for manufacturing three dimensional products. Some of the familiar prior art techniques for creating such products include, casting, extrusion, stereolithography and powder metallurgy. After the initial product is formed in the prior art, forming techniques, extractive techniques, chemical etching and additive or deposition techniques are often also performed to bring the product to final form.
Casting is usually performed by pouring a liquid, such as molten metal or plastic, into a mold and letting it cool and solidify. The metal takes the shape of the mold's interior surface as it solidifies. In extrusion semi-molten or molten plastic or hot metal is forced through an extrusion die which has a predetermined two dimensional shape. The extruded material takes the shape of the die and the shape of the die is transferred to the product through contact. In powdered metallurgy a batch of solid metal particles or powder is introduced into a mold where high temperature and pressure are applied to fuse or sinter the particles together. As is the case with casting, the end product assumes the shape of the mold's interior surface. In stereolithography an object is made by solidifying superposed layers of curable plastic resin until the complete object is built up.
After these initial objects are produced, forming techniques, extractive techniques, chemical etching, and additive or depositive techniques are often used to bring the product to the final form. Additional manufacturing techniques for making such objects include creating the products out of preformed component parts which are then joined by welding, soldering or brazing, or gluing.
However, many of these techniques have disadvantages. The molded form technique requires the mold be manufactured before the intended end product can be produced. In extractive techniques, much of the material is discarded causing waste of production materials. Metal fabrication by welding, soldering and brazing requires that the component parts be preformed before the final joining operation. In stereolithography individual layers may change their volume when solidifying causing stresses and deformation in the resultant product and materials are limited to a few plastic resins. In addition the specialized facilities needed for manufacturing are bulky and expensive.
A directional electrostatic accretion process employing acoustic droplet formation has been described in U.S. Pat. No. 5,520,715 by Oeftering, issued May 28, 1996 which addresses some of these issues. The process uses acoustically formed charged droplets of molten metal which are controlled by an acceleration electrode and deflection plates to build up a three dimensional product on a target substrate. The system is precisely controlled by a design workstation which has the parameters of the product to be built to insure the accuracy of the trajectory of each charged droplet. This process is certainly an improvement over prior processes because it requires less equipment that need not be retooled for every product desired to be reproduced, but it is limited in use because it must be operated in a vacuum or oxygen free atmosphere to eliminate the formation of an oxide skin on the free surface of the liquid metal. Formation of an oxide skin can impede ejection of metal droplets and absorb acoustic energy.
An oxygen free atmosphere can be created two ways, either operating in the vacuum of space or by enclosing the entire apparatus. Enclosing the apparatus requires additional large and complex machinery. Additionally, maintaining a precise depth of the pool of molten metal when the device is placed in a vacuum requires additional process steps not necessary when such a device is used in an atmospheric environment. Conventional displacement devices have been shown to be unreliable when used in a vacuum unoppsed by some external pressure means. Therefore the pool depth must be monitored and regulated using displacement means or an acoustic radiation pump.
It would therefore be desirable to build a manufacturing device, which requires fewer bulky parts, does not require retooling for each new part and which is capable of building three dimensional parts out of molten metal but which does not require the apparatus to be operated in a vacuum or an oxygen free atmosphere.
Further advantages of the invention will become apparent as the following description proceeds.